Wilder ends with the popular -er sound as many surnames like Thatcher, Fletcher, & Parker do. Wilder seems to match your unintentional nature theme. Wilder sounds like he could be a handful, but adventurous and wild & free. Wilder could also be shortened to the tamer Will if needed.

I think that what you are saying is that you want something aspirational, cool, maybe slightly 'badass' without being overtly nature-oriented and something that will keep your sibset from straying over to a caricature of toughness (Max and Steele).